The rolling bearing device for an axle is generally provided with an outer ring member that is attached to the vehicle body side and an inner ring member that is rotatably supported on the outer ring member so as to freely rotate around the bearing axis through two rows of rolling elements. This inner ring member is constituted by a hub shaft to which a wheel is attached and an inner ring bearing element that is fitted to the end portion of this hub shaft. The end portion of the hub shaft is deformed radially-outward to be caulked onto the end face of the inner ring bearing element so that the hub shaft is rotatably integrated with the inner ring bearing element. The hub shaft of this type is generally made of a steel material such as carbon steel. Such a steel material has a mixed structure of pearlite and pro-eutectoid ferrite.
When the end portion of the hub shaft is caulked onto the end face of the inner ring bearing element, minute cracks tend to occur on the caulked portion.
The present invention relates to the bearing device made of such a steel material having the mixed structure, and its object is to provide a bearing device that is free from minute cracks even when subjected to the caulking process, and a manufacturing method of such a bearing device.